Elucidation of the mechanism of specific export of protein through membranes in prokaryotic systems will provide information relevant to a phenomenon central to many biological processes, such as secretion and biogenesis of organelles. The proteins exported by E. coli fall into three classes. We will investigate the export of representative proteins from each class: 1) soluble periplasmic proteins, (specifically maltose-binding protein and arabinose-binding protein) 2) hydrophobic outer membrane proteins (specifically lambda receptor) and 3) the extracellular proteins (specifically colicin El). We propose experiments to detail the molecular mechanism of selection of the proteins for transport and the subsequent vectorial transfer through the membrane. This mechanism seems to fall within the general scheme of the signal hypothesis as proposed by Blobel and coworkers. The information required for transfer lies within the protein itself in a signal sequence which is removed by proteolytic cleavage during transfer through the membrane. We will characterize the nature of precursors of exported proteins which we synthesize in vitro: study the kinetics of processing in vivo (in the case of arabinose-binding protein) and determine whether processing plays a crucial role in activation and/or vectorial translocation of the exported proteins. We will look for specific membrane proteins which make up a "translocation apparatus" involved in the initial binding and guidance of nascent chains through the membrane. Biogenesis of the lambda receptor involves not only passage through the cytoplasmic membrane, but also insertion into the outer membrane. Investigations of this pathway may reveal a mechanism which differs in detail from that involved in export of soluble protein.